1. Field of the Invention (Technical Field)
The present invention relates to mortars and more particularly to a method and apparatus for isolating a linear shock while maintaining the alignment of a sensitive electronic pointing device for use on a barrel of mortar or similar device.
2. Background Art
During the firing of a large bore weapon a significant reaction force is imparted to the barrel and support structure. A support structure, which is required to travel a certain distance before absorbing the load, allows the barrel and its attached components to undergo an instantaneous high-g acceleration. A sensitive electronic pointing device, such as inertial measurement unit or inertial navigation unit, and its attachment structure would be, and has been, destroyed by this extreme acceleration and deceleration.
The present invention is an inertial isolation method and apparatus of a pointing device from the mortar barrel recoil travel accomplished effectively through it's mounting assembly. For example, the Ring Laser Gyro (RLG) which is an integral part of Honeywell's Tactical Advanced Land Inertial Navigator (TALIN™) pointing device requires a mortar mount assembly designed to provide a stable and protective cage parallel to the center line of the barrel. The mortar barrel moves approximately twelve inches (12″) under a high acceleration developing energy of approximately five hundred thousand foot pounds (500 k ft-lbs.) and then decelerates to a stop in less than 0.010 seconds when fired from a base plate in a free standing configuration. Most particularly, this mount needs to provide for the repeated firing of the mortar without realignment or mechanical adjustment while maintaining a zero ballistic force vector on the pointing device.
Presently the prior art PDMAs (pointing device mounting assembly) cannot withstand the recoil acceleration force while attached to a 120 mm mortar barrel when fired while mounted on a mortar weapon, such as the M9, base plate in the dismounted configuration. The present prior art PDMA experiences catastrophic failure of the steel mounting plates due to stress in excess of the bending moment of the material of their construction. This force exceeds the isolators travel limit and transfers the shock load into the RLG pointing device and causes internal physical damage.
The pointing device mounting assembly currently in use by the United States Army consists of two separate steel plates mounted to the mortar barrel with a pointing device cage suspended between them on an array of rubber isolators. This design provides a level of shock isolation for the pointing device only when fired from a non-recoiling platform (M-1064 vehicle mounted as opposed to free standing base plate). Problematic with the present design is the fact that plate alignment during attachment to the barrel is not easily indexed and this design cannot be used on the mortar barrel when fired from a base plate dismounted configuration due to the high gravity (g) load caused by the force of acceleration over the seating travel distance. This configuration has in the past bent and broken the steel plates and exceeded the shock isolation limits to the RLG pointing device.
Others have tried to solve the problem by designing a mounting platform for the RLG pointing device which combines the mortar barrel bi-pod support buffers in an assembly which attaches to the barrel and allows the mortar barrel to recoil while separating the RLG pointing device from recoil force through a shaft and bearing assembly on the bi-pod attachment collar.
The attempt in the prior art to design a mechanical force vector isolation system for the RLG pointing device fails to address the requirement for symmetry and even distribution of force throughout its design. The prior art design produces an unsupported moment arm which multiplies the recoil force vector rather than separating it. This causes the shaft and bearing assembly to seize and transfer the recoil force into the bi-pod attachment collar causing it to slip. The increased force applied to the offset design transmits a multiplied force into the RLG pointing device through the unsupported moment arm. The magnitude of the forces has caused the materials of construction in this prior art design to fail.
A prior art device is described in U.S. Pat. No. 4,336,917, which does not use guide rails and bearings for linear shock isolation and to maintain position alignment. It uses gas driven pistons and gas accumulator/controllers that are sensor-controlled to maintain position during shock and vibration. Another prior art device is described in U.S. Pat. No. 6,814,179, which also does not use guide rails and bearings for linear shock isolation and to maintain position alignment. It uses shock isolators that are comprised of rubber and polyurethane foam to absorb shock and vibration.
The present invention separates the sensitive electronic pointing device from the force vector during the specific impulse of firing by suspending it in inertial space while at the same time maintaining near perfect alignment with the bore axis of the barrel. This invention solves the problem of inertial isolation by providing a support structure which maintains the linear position of the shaft/bearing interface in a parallel plane with the axis of travel of the mortar barrel. The shaft/bearing support structure also distributes the firing loads evenly along the shaft during the recoil action and prevents the weight of the RLG pointing device from deflecting the shaft bearing assembly out of plane during the travel stage of the recoil.